Traditionally, in a vehicle drive apparatus mounted on a hybrid vehicle as a motor-driven vehicle which transmits a part of engine torque to a generator (generator motor) and the remainder to driving wheels, it has a planetary gear unit as a differential rotation unit having a sun gear, a ring gear and a carrier in which the sun gear is joined to the generator, the ring gear and a drive motor are joined to the driving wheels, the carrier is joined to the engine, and the rotation outputted from the ring gear and the drive motor is transmitted to the driving wheels to generate driving force.
In the hybrid vehicle, an inverter is disposed between the drive motor and a drive motor control apparatus in which the inverter is driven in accordance with a drive signal sent from the drive motor control apparatus, receives the power of direct current from a battery to generate the currents of phase U, phase V and phase W, and supplies the current of each phase to the drive motor. Therefore, the inverter has multiple transistors as six switching devices, for example, in which the individual transistors are formed in a unit by one pair each to configure a transistor module (IGBT) for each phase. Thus, when the drive signal is sent to the individual transistors in a given pattern, the transistors are turned on and off to generate the current of each phase.
Then, a drive motor rotational speed sensor detects drive motor rotational speed to be the rotational speed of the drive motor, and controls the drive motor torque to be the torque of the drive motor, for example, based on the drive motor rotational speed (for example, see JP-A-2002-12046).
However, in the traditional vehicle drive apparatus, when the amount of battery power remaining is small or when a driver presses down an accelerator pedal, for example, the engine is started, whereas when the amount of battery power remaining is large or when the driver releases the accelerator pedal, the engine is stopped. Large vibrations are generated in the engine rotational speed NE to be the rotational speed of the engine as the engine is started and then stopped, and these vibrations sometimes arouse uncomfortable feelings in the driver.
FIG. 2 is a time chart illustrating the state of the engine rotational speed when the engine is started and then stopped in the traditional vehicle control apparatus.
In the drawing, when an engine start request is outputted and the engine start request signal Sg1 is set to high level at timing t1 in the vehicle control apparatus, the generator sets the engine rotational speed NE to the value required to start the engine and performs fuel injection and ignition. Then, it increases the engine target rotational speed NE* that shows the target value of the engine rotational speed NE in a pattern shown in the drawing as well as the engine rotational speed NE follows it and goes to high.
Subsequently, the engine start request is not outputted and the engine start request signal Sg1 is set to a low level at timing t2 in the vehicle control apparatus. Also, an engine stop request is outputted and the engine stop request signal Sg2 is set to a high level, and then fuel injection and ignition are stopped. After timing t2, the engine target rotational speed NE* is decreased in a pattern as shown in the drawing as well as the engine rotational speed NE follows it and set to low.
In the meantime, the engine rotational speed NE increases over time from timing t1 to t2, whereas the engine rotational speed NE decreases over time after timing t2; the engine rotational speed NE is reversed at timing t2.
However, the vehicle drive apparatus has the predetermined number of unique vibrations, and a resonance range based on the number of unique vibrations is formed between the engine rotational speeds NE1 and NE2 that are lower than the idle rotational speed NEid. Thus, when the engine rotational speed NEx takes the value within the resonance range at timing t2, large vibrations are generated in the engine rotational speed NE as the engine rotational speed NE is reversed.
When the amplitude of the engine rotational speed NE becomes greater, teeth of various gears and splines which configure the vehicle drive apparatus hit against each other to cause the gears and splines to deteriorate from abrasion. In this case, the durability of the vehicle drive apparatus is decreased as well as run feelings in running the hybrid vehicle are reduced.